The invention relates to a transmission lines arrangement comprising a first plurality of transmission lines, each transmission line having an effective characteristic impedance and comprising a second plurality of transmission lines, said first plurality of transmission lines being coupled to a plurality of switching elements, the plurality of switching elements redirecting an input signal from one transmission line of the first plurality of transmission lines to at least one transmission line of the second plurality of transmission lines.
In modern communication circuits operating at GigaHerz (GHz) frequencies e.g. crosspoint matrix switching circuits, most often a circuit connection between portions of the circuits could be considered to behave like a transmission line having a characteristic impedance. As examples of transmission lines could be cited telephone lines, wave guides, connections between computers.
In such a crosspoint matrix switching circuit an input signal could be redirected via switching elements to at least one output, e.g. in a broadcast mode the input signal is redirected to all the outputs simultaneously. Usually such a crosspoint matrix circuit may be symbolically represented as a bi-dimensional array comprising n horizontal transmission lines i.e. the first plurality of transmission lines and m vertical transmission lines i.e. the second plurality of transmission lines. At any crosspoint between any horizontal transmission line and any vertical transmission line there is a switch element for redirecting a signal coming e.g. on a horizontal transmission line to at least one vertical transmission line. Accordingly the switches could be considered to be distributed on a horizontal line and on a vertical line. A switch element is an electronic device having an input and an output and being controlled by a binary signal. When the binary signal is in one of it""s two possible states e.g. 0 then a signal that is present at the input of the switch is present at the output of the switch, too and the switch is ON. Otherwise, when the binary signal is in the other state e.g. 1 then the signal present at the input of the switch is not present at the output of the switch and the switch is OFF. For each horizontal line, one or more, or possibly all of the switches may be ON. For each vertical line, only one switch is ON. It is also further considered that each of the horizontal transmission lines is driven by a driver having an input and an output. The input of the driver adapts to an input impedance connection e.g. 50 ohm and it""s output adapts to a characteristic impedance of the horizontal line. Because the horizontal transmission lines are coupled to the switching elements the input impedance of said switching elements influences the characteristic impedance of the transmission line, the characteristic impedance becoming an effective characteristic impedance. Therefore in order to maintain the integrity of an input signal independent with respect to effective characteristic impedance of the horizontal transmission line, said driver must be adapted to the effective characteristic impedance.
Such a solution is known from U.S. Pat. No. 6,175,239B1. According to the known solution in a communication system comprising transmission lines coupled to the outputs of buffers it is provided an apparatus that determines the effective impedance of the transmission lines. The information provided by the apparatus is then used to control the output impedance of the buffers. The apparatus is further used in a computer network, the process of determining the effective characteristic impedance occurring any time a computer is started up. It should be mentioned here that the above mentioned apparatus is not applicable to a crosspoint matrix switching circuit because some of the switches coupled to the horizontally distributed transmission lines are ON and the other are OFF influencing the characteristic impedance of the horizontal transmission line. The tuning process e.g. adapting the output impedance of the buffer to the effective impedance of the transmission line should take place any time when at least one switch changes it""s state. Because the tuning process is time consuming, in circuits working at GHz frequencies the method is not very well applicable. Furthermore, because the horizontal lines are coupled all the time only to the switches a simpler tuning method should apply.
It is therefore an object of the present invention to provide a transmission lines arrangement suitable to be used in a crosspoint matrix circuit. It is another object of the present invention to provide a transmission lines arrangement with a delay of any input signal significantly independent with respect to the path i.e. the way from an input to a selected output.
In accordance with the invention this is achieved in a device as described in the introductory paragraph characterized in that each of the switching elements of the plurality of switching elements have a relatively high input impedance in comparison with the effective characteristic impedance and a relatively high output impedance in comparison with the effective characteristic impedance, each transmission line of the first plurality of transmission lines being further coupled to an impedance that is substantially equal to the effective characteristic impedance of said transmission line.
Considering that the switching elements are positioned at a distance D from each other then each section of a transmission line is characterized by a capacity Cd. In order to minimize the influence of the input impedance of the switching elements their input capacitance should be much lower than Cd.
Furthermore, their input resistance must be much higher than the characteristic impedance of the transmission line such that in the worst case i.e. when all the switches coupled to a transmission line are ON the overall impedance of the transmission line to be substantially uninfluenced. When the transmission line arrangement is realized in a chip the characteristic impedance and the capacitance of a portion with the length D are known and an overall appreciation of the effective characteristic impedance could be made. Under these circumstances a resistance that equals the effective transmission line impedance is coupled at the end of the transmission line.
The output of the switching elements are coupled to the vertical transmission lines and at one moment only one of the switching elements is ON. Under these circumstances the output of the other switching elements must not influence the transmission of a signal through the transmission line i.e. none of the switching elements must be a receiver for the signal. As a matter of consequence the output impedance of the switching element must be much greater than the effective characteristic impedance of the transmission line.
In an embodiment of the invention the transmission line arrangement comprises as switching elements switcheable voltage to current (V-I) converters. The switcheable V-I converters have relatively high input impedance and relatively high output impedance fulfilling the conditions for the switching elements as they were previously presented. It should be mentioned here that the switcheable V-I converters have been chosen for illustrative purposes as an example of a switching element, skilled persons in the art being able to use other switching elements having high input impedance and high output impedance, such as transimpedance amplifiers.
In another embodiment of the invention the plurality of switching elements are differential. This feature reduces crosstalk to the substrate where the transmission lines are laid. Otherwise this influence is greater when single ended signals are used.
In another embodiment of the invention the first plurality of transmission lines is further coupled to a first plurality of input buffers for adapting to an input signal and the second plurality of transmission lines is further coupled to a second plurality of output buffers for transmitting the redirected signal. The buffers are useful in order to adapt the transmission lines to an external input connection e.g. 2xc3x9750 ohm the output impedance of the buffers being significantly equal to the characteristic impedance of the transmission lines.
In another embodiment of the invention the transmission line arrangement is characterized in that each of the transmission lines of the first plurality of transmission lines comprises portions of transmission lines and each of the transmission lines of the second plurality of transmission lines comprises portions of transmission lines. The transmission line arrangement further comprises a third plurality of delay elements coupling the first portions of transmission lines and a fourth plurality of delay elements coupling the second portions of the transmission lines. The total delay of a signal passing through any line of the first plurality of transmission lines and any line of the second plurality of transmission lines is independent with respect to which one of the first plurality of portions and which one of the second plurality of portions it passes through.
It should be observed that when very high frequency i.e. GHz range input signals pass through relatively long transmission lines the input signal deteriorates and the overall delay of the input signal through the arrangement depends on the signal path i.e. the transmission lines passing through. In order to obtain a significantly equal delay the transmission lines comprise portions of transmission lines coupled to delay elements such that the delay of an input signal through the arrangement is substantially independent with respect to the path through the arrangement.